FULL PRODUCT VERSION :
Java(TM) SE Runtime Environment (build 1.6.0-rc-b95)


A DESCRIPTION OF THE PROBLEM :
The LAFS, which are responsible for painting the background content area of a JTabbedPane should respect the color set on the background of the widget. I see this behavior in these UIs:
1. Metal: uses UI color
2. Motif: respects background color
3. GTK: uses UI color
4. Windows, unknown untested

This code in BasicTabbedPaneUI shows the coloring priorities:
            Color color = UIManager.getColor("TabbedPane.contentAreaColor");
            if (color != null) {
                g.setColor(color);
            }
            else if ( selectedColor == null || selectedIndex == -1 ) {
                g.setColor(tabPane.getBackground());
            }
            else {
                g.setColor(selectedColor);
            }

The ui color comes first then the selected color. I would think the true order would be:
1. Selected color
2. Background color
3. UI color

#1 could be #2 in this list, Im assuming that there is a way to set the color.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Run this code, try with different LAFs see which ones result in a green background color:
import javax.swing.*;
import java.awt.*;

public class BackgroundJTabbedPane implements Runnable{

    public void run(){

        JTabbedPane jtp = new JTabbedPane();
        jtp.setBackground(Color.green);
        for(int i = 0; i < 10; i ++){
            JPanel jp = new JPanel();
            jp.setOpaque(false);
            JButton jb = new JButton(String.valueOf(i));
            jp.add(jb);
            jtp.addTab(String.valueOf(i), jp);
        }

        JFrame jf = new JFrame();
        jf.add(jtp);
        jf.setSize(500,500);
        jf.setVisible(true);
    }



    public static void main(String ... args){
        try{
int i = Integer.parseInt(args[0]);
            if(i == 0) UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
            else if(i == 1) UIManager.setLookAndFeel("com.sun.java.swing.plaf.motif.MotifLookAndFeel");

        }
        catch(Exception x){}
        SwingUtilities.invokeLater(new BackgroundJTabbedPane());
    }



}

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
a nice green background
ACTUAL -
dependant upon LAF, motif is the only one with a nice green background

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
import javax.swing.*;
import java.awt.*;

public class BackgroundJTabbedPane implements Runnable{

    public void run(){

        JTabbedPane jtp = new JTabbedPane();
        jtp.setBackground(Color.green);
        for(int i = 0; i < 10; i ++){
            JPanel jp = new JPanel();
            jp.setOpaque(false);
            JButton jb = new JButton(String.valueOf(i));
            jp.add(jb);
            jtp.addTab(String.valueOf(i), jp);
        }

        JFrame jf = new JFrame();
        jf.add(jtp);
        jf.setSize(500,500);
        jf.setVisible(true);
    }



    public static void main(String ... args){
        try{
int i = Integer.parseInt(args[0]);
            if(i == 0) UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
            else if(i == 1) UIManager.setLookAndFeel("com.sun.java.swing.plaf.motif.MotifLookAndFeel");

        }
        catch(Exception x){}
        SwingUtilities.invokeLater(new BackgroundJTabbedPane());
    }



}
---------- END SOURCE ----------